The depth to which martensite can be obtained by quenching (how readily a steel hardens). It increases when alloying elements such as Mn, Cr and Mo shift the nose of the TTT diagram to longer times.
Japanese name: 焼入れ性
Hardenability appears in 2 practice questions across 2 topics, including Heat treatment and metallic materials, Materials processing and joining.
Hardenability expresses how deep martensite can be produced by quenching (how readily the steel hardens). The maximum hardness of the martensite produced (the as-quenched hardness), on the other hand, is determined mainly by carbon content, with little influence from alloying elements. So adding alloying elements to raise hardenability makes the hard layer deeper without changing the maximum surface hardness much. A helpful summary is that carbon content mainly sets hardness, while alloying elements and austenite grain size mainly set hardenability.
Hardenability is evaluated with the Jominy test (end-quench test). A round bar about 25 mm in diameter and 100 mm long is austenitized and then cooled by spraying water onto one end, so the cooling rate varies continuously with distance from that end. After cooling, flats are ground on the side and hardness is measured at each distance from the end, giving the Jominy curve. The more slowly hardness falls, the higher the hardenability; H steels with guaranteed hardenability (such as SCM435H) are specified by upper and lower bands of this curve.
The critical diameter expresses hardenability in terms of part size: it is the largest diameter of a round bar whose center becomes 50% martensite. The value for an ideal quench with infinite cooling power is called the ideal critical diameter, and it can be estimated by finding a base value from carbon content and grain size and multiplying by factors for each alloying element. The phenomenon in which even the same steel hardens less readily in thicker parts, because the center cools more slowly, is called the mass effect, and it is larger in low-hardenability carbon steels. Cooling power also depends on the quenchant, such as water, oil or aqueous polymer solutions.
Alloying elements such as Mn, Cr, Mo and Ni, and tiny amounts of B around 0.001–0.003%, raise hardenability. A coarser austenite grain size raises hardenability by reducing the grain boundary sites for nucleation, but it lowers toughness. Thicker parts need steels with higher hardenability, but too much hardenability makes quench cracking and distortion more likely, and in welding the heat-affected zone becomes hard and prone to cracking. The basic principle is to choose a steel with the minimum hardenability that still hardens to the required depth.
Answer: B. The nose of the TTT diagram moves to longer times, so martensite forms more easily even with slow cooling
Hardenability is the depth to which martensite can be obtained by quenching (how readily the steel hardens). Alloying elements such as Mn, Cr and Mo retard the pearlite and bainite transformations of austenite and move the nose of the TTT and CCT diagrams to longer times. This lowers the critical cooling rate, so martensite forms more easily at the center of thick sections or with slower cooling such as oil quenching. If the nose moved to shorter times, the steel would harden less readily. Most alloying elements actually lower the Ms temperature, so that is not the reason for improved hardenability. Hardenability is evaluated with the Jominy end-quench test.
No. Higher hardenability lets hardening reach deeper, but the maximum hardness of martensite is set mainly by carbon content. At the same carbon content, alloy steels and carbon steels have almost the same as-quenched surface hardness.
It is the phenomenon in which even the same steel hardens less readily in thicker parts because the interior cools more slowly. It is large in low-hardenability carbon steels and small in alloy steels. To harden thick parts through to the center, choose a steel with high hardenability.
B segregates to austenite grain boundaries and suppresses the nucleation of ferrite there. It is effective at about 0.001–0.003%, and it is used together with elements such as Ti so that it does not combine with nitrogen.
Hardenability is the depth to which martensite can be obtained by quenching (how readily the steel hardens).
Adding alloying elements such as Mn, Cr and Mo to steel imp…The carbon equivalent converts the effect of each alloying element on hardenability into an amount of carbon and sums them.
The carbon equivalent used as a guide to the weldability of…Martensite Pearlite Carbon equivalent Heat-affected zone Hall–Petch relationship Precipitation hardening Tempering